1. Field of the Invention
The present invention relates generally to wireless telecommunications and more particularly to a method for determining the execution time and handoff direction of a hard handoff by a base station controller (BSC) in a code division multiple access (CDMA) mobile telecommunication system.
2. Description of the Related Art
A cellular mobile telecommunication system divides a total service area into a plurality of cells where each cell is serviced by a base transceiver station (BTS). By centralizing the base transceiver stations in each cell, subscribers are able to communicate with each other as they move between cells. That is, the cellular system configuration makes it possible for a mobile station to continue a call when the mobile station moves out of a cell being served by one BTS and into another cell being served by another BTS.
FIG. 1 illustrates the structure of a cellular system using conventional CDMA technology. As illustrated in FIG. 1, the cellular system includes a mobile station (MS) 40 receiving mobile telecommunication services, base transceiver stations (BTSs) 30, 31 providing the MS 40 with the mobile telecommunication services, a base station controller (BSC) 20 controlling the BTSs 30, 31 and a mobile switching center (MSC) 10 connecting the BSC 20 to the public switched telephone network (PSTN) 11. The MSC provides services by receiving information about the MS 40 from a home location register (HLR) 12, and a visitor location register (VLR) 13.
In the cellular system described in FIG. 1, a link from BTSs 30, 31 to MS 40 is commonly referred to in the art as a forward link, and a link from MS 40 to BTSs 30,31 is commonly referred to as a reverse link. All forward links associated with a particular BTS share the same pseudorandom noise (PN) sequence offset. The PN offset is transmitted through a pilot channel as one of the forward channels, referred to as a pilot signal. The pilot signal serves as an identifying signal distinguishing one BTS from another.
A cellular system can employ techniques to accommodate additional users. One such technique is cell division. In cell division a cell is trisected by separating three (3) 120 degree antennas at a 120xc2x0 spacing to each other, thereby dedicating an antenna to each of the three sectors. A mobile station recognizes each sector antenna as a separate BTS.
Cellular systems and personal communication services (PCS) systems using CDMA technology provide various types of handoffs to guarantee the continuance of a call. Handoffs allow the continuance of a call by establishing a new channel within a short period of time from losing a currently established channel due to movement of the mobile station from one service area to another. The time duration of losing one channel and establishing another is short enough such that a user will find it difficult to detect that a handoff has occurred.
When the capacity of a cellular system must be upgraded to accommodate additional users, the cellular system assigns additional frequencies (i.e., channels) to the BTS. A frequency assignment is commonly referred to as an (FA). This situation often occurs in an urban environment where a city center requires many FAs due to the dense urban population. By contrast, a suburban setting would require fewer FAs.
In a conventional cellular system, when a mobile station, which is communicating via a specific frequency channel (i.e., FA), moves to a neighboring service area being served by another BTS which does not service the specific frequency channel, or the BTS does not have an idle traffic channel mobile station recognizes that it is moving into a region being serviced by a neighboring BTS by receiving a pilot signal via a continuously transmitted pilot channel of the neighboring BTS. In a mobile assisted hard handoff, conventional systems must install a separate pilot receiver in the mobile station to identify pilot signals from a neighboring BTS to support the hard handoff.
A hard handoff method using pilot signals is disclosed in U.S. Pat. No. 5,594,718, xe2x80x9cMethod and apparatus for providing mobile unit assisted hard handoff from a CDMA communication system to an alternative access communication systemxe2x80x9d.
In a mobile assisted hard handoff, the mobile station detects whether the strength of the pilot signal of a neighboring BTS is satisfactory using the pilot transmitter/receiver specially installed in the mobile station. If the mobile station determines that the strength of the pilot signal is satisfactory, it then calculates handoff decision parameters, such as a handoff execution time, to demand a handoff to a BSC. That is, the conventional system describes a mobile assisted handoff whereby the mobile station demands a handoff to the BSC at an execution time when the strength of the pilot signal of the neighboring BTS is equal to a pre-determined power level, T_ADD.
If the transmission frequency of the neighboring BTS detected by the mobile station is being maintained as a dummy pilot solely for the purpose of hard handoffs, the BSC decides that, despite the mobile""s demand for a handoff, there is no corresponding communication resources available in the neighboring BTS. The BSC instead completes an inter-frequency hard handoff by performing a hand-down to a common frequency that both the currently servicing BTS and neighbor BTS commonly use (i.e., a hand-down allowance frequency). The BSC then performs an inter-cell soft handoff to the neighboring BTS.
The present invention discloses a method for determining the execution time and handoff direction of an inter-frequency hard handoff in the case where a mobile station is moving between two base station transceivers have different frequency allocations (i.e. different FAs).
The method of the present invention generally comprises the steps of: receiving a pilot strength measurement message (PSMM) from the mobile station by the base station controller (BSC), the PSMM including a plurality of pilot signals each having a pilot signal strength; upon receiving the PSMM by the BSC, determining whether all sub-cells with which the mobile station is currently in communication are border frequency sub-cells; if it least one sub-cell is determined not to be a border frequency sub-cell, a general handoff is performed; otherwise if all sub-cells are determined to be border frequency sub-cells, periodically ordering the PSMM from all sub-cells with which the mobile station is currently in communication by the BSC; analyzing a received PSMM transmitted from the mobile station in response to the PSMM order issued by the BSC to determine whether the pilot signal strengths received from all sub-cells are lower than a predetermined power level, T_DOWN, and activating a timer set to a predetermined time, T_TDOWN if said pilot signal strengths received from all sub-cells are lower than a predetermined power level; if the timer expires without interruption during the time interval T_TDOWN ordering the mobile station to execute an intra-cell inter-frequency hard handoff to a common frequency that all base transceiver stations service.
The present invention is described in the context of a mobile telecommunication system having a sectorized cell structure, however, it is more generally applicable to systems having configurations which do not employ such a structure.
An advantage of the present invention is that it is implemented exclusively in software without the need for additional hardware.